Unveiling the origin of the large coercivity in (Nd, Dy)-Fe-B sintered magnets

Abstract

Abstract Nd-Fe-B-based permanent magnets are widely used for energy conversion applications. However, their usage at elevated temperatures is difficult due to the relatively low coercivity ( H c ) with respect to the anisotropy field ( H A ) of the Nd 2 Fe 14 B compound, which is typically 0.2 H A . In this work, we found that the coercivity of an (Nd 0.8 Dy 0.2 )-Fe-B sintered magnet could reach 0.4 H A , which was twice as high as the H c / H A of its Dy-free counterpart. Detailed microstructural characterizations, density functional theory and micromagnetic simulations showed that the large value of coercivity, H c = 0.4 H A , originated not only from the enhanced H A of the main phase (intrinsic factor) but also from the reduced magnetization of the thin intergranular phase (extrinsic factor). The latter was attributed to the dissolution of 4 at.% Dy in the intergranular phase that anti-ferromagnetically coupled with Fe. The reduction in the magnetization of the intergranular phase resulted in a change in the angular dependence of coercivity from the Kondorsky type for the Dy-free magnet to the Stoner–Wohlfarth-like shape for the Dy-containing magnet, indicating that the typical pinning-controlled coercivity mechanism began to show nucleation features as the magnetization of the intergranular phase was reduced by Dy substitution.